An oil well is drilled with a typical tricone drill bit and assembly with threads to the bottom of a string of drill pipe. It has a hollow threaded member with an axial flow passage within the assembly to direct drilling fluid, usually known as drilling mud, out through a number of openings to wash cuttings away from the cones which form the cutting. Rotation of the drill string and attached drill bit is from the surface of the earth. Teeth on the drill bit are rotated against the face and wall of the well borehole thereby cutting the earth formations as the drill bit rotates, thereby advancing the borehole. The drill bit has three cones mounted for contact against the face of the borehole. Each cone rotates its teeth with the rotation of the drill string, thereby cutting the borehole. Drill bit wear predominately occurs at the teeth. As the teeth wear, the penetration rate declines and the drill bit has to be replaced.
Cones and their teeth have a specified wear rate. Better performance has been obtained by enhancing the wear characteristics of the cone teeth, or "inserts". Inserts are positioned within each cone hole. The inserts are harder than the metal cone. Most inserts are formed of various carbides, extremely hard materials. Primary contact and wear of the insert occurs at the exposed outer end of the insert. Greater protection yet has been provided from industrial grade diamonds. The optimum wear protection is obtained by the attachment of a cap or crown of industrial grade diamond which covers the exposed insert end. This type of crown is often known as a polycrystalline diamond compact (PDC). The carbide insert body is not pure WC, but is preferably granules of WC which are interspersed with an alloy which binds the WC particles. The preferred alloy is a cobalt based alloy. Likewise, the PDC crown is not a layer of pure diamond, but is an agglomeration of diamond particles held together with a binding metal matrix. Again, this binding material is typically a cobalt based alloy. The PDC cap or crown is normally attached to the WC insert body by ultra high pressure and heat. The sintering material may also contain a substantial amount of cobalt. Specific materials are notable. The insert body is usually WC which is harder than other common metal carbides. While other metal carbides will work in some degree, WC is the common and preferred material. In like fashion, the binding alloy is usually about 15% or so of cobalt in the alloy matrix holding the WC particles together. A common alloy with WC is sold as the model 374 by Roger's Tool Works and includes an alloy having as low as 6% up to about 15% cobalt with other metals of less significance. The cobalt is the most significant part of the alloy as will be discussed below.
In prior art, elements of the insert are typically manufactured separately and subsequently assembled. The manufacture of the components is usually by sintering under very high temperature and very high pressure. This requires equipment which is physically large, and which is also very expensive to manufacture, maintain and operate. In addition, the high temperature can induce adverse chemical and physical changes in insert components, which will be discussed in subsequent sections of this disclosure.
As discussed in U.S. Pat. No. 5,011,515, composite polycrystalline diamond compacts, PDC, have been used for industrial applications including rock drilling and metal machining for many years. As an example, the composite compact consisting of PDC and sintered substrate are affixed as insert elements in a rock drill bit structure. One of the factors limiting the success of PDC is the strength of the bond between the polycrystalline diamond layer and a sintered metal carbide substrate. It is taught that both the PDC and the supporting sintered metal support substrate must be exposed to high pressure and high temperature, for a relatively long period of time, in order to achieve the desired hardness of the PDC surface and the desired strength in the bond between the PDC and the support substrate.
U.S. Pat. No. 3,745,623 (reissue U.S. Pat. No. 32,380) teaches the attachment of diamond to tungsten carbide (WC) support material with an abrupt transition there between. This, however, results in a cutting tool with a relatively low impact resistance. Due to the differences in the thermal expansion of diamond in the PDC layer and the binder metal alloy used to cement the metal carbide substrate, there exists a shear stress in excess of 200,000 psi between these two layers. The force exerted by this stress must be overcome by the extremely thin layer of cobalt which is the common or preferred binding medium that holds the PDC layer to the metal carbide substrate. Because of the very high stress between the two layers which have a flat and relatively narrow transition zone, it is relatively easy for the compact to delaminate in this area upon impact. Additionally, it has been known that delamination can also occur on heating or other disturbances in addition to impact. In fact, parts have delaminated without any known provocation, most probably as a result of a defect within the interface or body of the PDC which initiates a crack and results in catastrophic failure. See also Patent 4,811,801.
One solution to the PDC-substrate binding problem is proposed in the teaching of U.S. Pat. No. 4,604,106. This patent utilizes one or more transitional layers incorporating powdered mixtures with various percentages of diamond, tungsten carbide, and cobalt to distribute the stress caused by the difference in thermal expansion over a larger area. A problem with this solution is that "sweep-through" of the metallic catalyst sintering agent is impeded by the free cobalt and the cobalt cemented carbide in the mixture. In addition, as in previous referenced methods and apparatus, high temperatures and high pressures are required for a relatively long time period in order to obtain the assembly disclosed in U.S. Pat. No, 4,604,106. Pressures and temperatures are such that, using mixtures specified, the adjacent diamond crystals are bonded together.
U.S. Pat. No. 4,784,023 teaches the grooving of polycrystalline diamond substrates but it does not teach the use of patterned substrates designed to uniformly reduce the stress between the polycrystalline diamond layer and the substrate support layer. In fact, this patent specifically mentions the use of undercut (or dovetail) portions of substrate ridges, which solution actually contributes to increased localized stress. Instead of reducing the stress between the polycrystalline diamond layer and the metallic substrate, this actually makes the situation much worse. This is because the larger volume of metal at the top of the ridge will expand and contract during temperature cycles to a greater extent than the polycrystalline diamond, causing the composite to fracture at the interface. As a result, construction of a polycrystalline diamond cutter following the teachings provided by U.S. Pat. No. 4,784,023 is not suitable for cutting applications where repeated high impact forces are encountered, such as in percussive drilling, nor in applications where extreme thermal shock is a consideration.
By design, all of the cutting surfaces consisting of "conventional" alloys which are disclosed in the above references are "hard" in that they are abrasion and erosion resistant. This is particularly true for PDC material which is also quite brittle and subject to fracturing upon impact. Because of the brittleness and overall hardness, it is not practical and economical to machine surfaces of tools, bearings and the like made of PDC in the manufacturing process for these devices. Alternately, the PDC surfaces are preferably "molded" or performed using techniques taught in U.S. Pat. No. 4,662,896. Brittleness and fracture resistance are also noted in Patent 4,811,801.
The paper "Iron Aluminum-Titanium Carbide Composites by Pressureless Melt Infiltration-Microstructure and Mechanical Properties" by R. Subramanian et al (Scripta Materialia, Vol. 35, No. 5, pp. 583-588, 1996, Elsevier Science Ltd.) discloses a technique for fabricating wear resistant material which does not require high pressure. Conversely, a mixture of powdered components is placed in a dynamic vacuum of 10-4 Pa, heated to a temperature of 1450 for about one hour. The binding component melts and flows into the interstitial voids of the wear resistant component. Vacuum equipment is obviously required to fabricate the wear resistant material.
U.S. patent application Ser. No. 08/517,814 which was filed on Aug. 22, 1995 by the present inventor discloses apparatus and methods for forming composite inserts at relatively low temperature and pressure. The composite insert can be assembled by brazing a separately sintered wear component to a support component, or by sintering the wear component directly onto the support component. The wear surface consists of a sintered mixture or "cermet" of crystalline material, metal and/or metallic carbides. These alloy materials are selected to minimize the sintering heat and temperature requirements. In a preferred embodiment, the wear surface material created by sintering consists of a mixture of abrasion resistant crystals, preferably diamond crystals, and a metal, which partially transforms during sintering to metal carbide, is a cemented diamond compact containing 60% or more diamond by volume, but lacking diamond to diamond bonding. Due to the high metal content and the short time of sintering, not all of the metal is reacted with the abrasion resistant material. The metal which is not reacted is then free to form a matrix in which the abrasion resistant material is suspended. This metal matrix is responsible for the enhanced ductility and fracture toughness of the material. The end result is a material with comparable abrasion and erosion properties to conventional, prior art materials, but the cermet is less costly to produce, has better impact resistance, and is more easily formed. A mold or cast is required to contain the wear resistant component in the low temperature cermet alloy during the low temperature and low pressure sintering operation. Disclosed means for heating are a simple torch, an induction oven, a source of infrared light, a laser source, a plasma, or a resistive heating oven. Attempts are made to use materials with matching thermal coefficients to minimize stress between the cermet and support components and stress within the cermet, although it is still sometime preferable to anneal the final product to reduce stress in the finished product.
The parent application for this continuation-in-part discloses apparatus and methods for forming sintered components of alloys using microwave energy as a heat source, wherein the alloys are "conventional" in that they were previously used only in high temperature and high pressure sintering processes The insert body and the insert wear crown can be sintered as an integral insert within a mold, or can be sintered separately and subsequently joined by brazing as previously discussed. As an important additional advantage, the mold to contain the raw materials can even be completely eliminated by the use of a sacrificial binding agent such as wax prior to sintering. The microwave energy source permits the sintering process to be completed in a relatively short period of time, and at very low pressure. Temperature can also be controlled. If sintered as a unit, migration of cobalt within the various components is negligible due to the relatively short sintering time required. The disclosure also teaches that smaller grain sizes can be obtained without the use of grain growth inhibitor, which can adversely affect the insert in other ways. Stress concentration at the interface of insert components is still present, although markedly reduced if the insert is sintered as a unit. Stress concentration at the interface of components assembled after sintering can be significant.
There is a delicate balance to be obtained in the finished wear product between hardness and resiliency. If materials are harder, they are lacking in resilience, and if they are resilient, they are lacking in hardness. As discussed previously, composite materials such as a wear resistant crown and an insert body of differing material yield high quality inserts. However, the composite materials are all different and therefore have contradictory criteria meaning they have different measures of hardness, different resiliency, different rates of thermal expansion, and different measures of shock resistance. A representative insert will be described which utilizes a central steel shank or body. The body, in turn, is covered with the WC abrasive resistant material. Separalely, a PDC crown is made at another location and then this PDC layer is brazed to the partly finished WC clad steel shank. Prior art manufacturing is typically by high pressure high and temperature sintering, sometimes known as "HPHT" sintering. While the finished product is quite successful, there are, however, problems that arise because of the dissimilarities in the various materials making up the finished device. In one aspect, the sintering process mandates that the components be made separately and later joined. This leads inevitably to transverse planar regions which localize possible stress failure. In a typical insert, the PDC crown is brazed by a braze region which measures only about 0.001 to about 0.004 inches thick. Moreover, this thin region of braze material must secure dissimilar materials together so that there are stress levels in this braze region which are detrimental to long life. Even if the stress is relatively minimal by careful manufacture, the drill bit is used in elevated temperatures so that stress concentrations can again build up which are not common at ambient temperatures. Regrettably, the failure mode of many inserts is fracture along the braze plane so that part or all of the PDC crown will break off.
This type of insert defies stress relieving by annealing using some prior art teachings. For instance, in the manufacture of glass and other relatively brittle materials, the finished product can be gently heated to a relatively high temperature for a long period of time and then gently cooled over a long time interval to obtain some internal stress relief. That is not so readily effective for composite drill bit inserts. There is a problem with migration of cobalt between differing elements or regions of the composite insert. Suffice it to say, the cobalt levels in different regions vary because different quantities of cobalt are required to provide the bonding matrix holding the various different particles together. The cobalt concentration in the PDC layer is different from the cobalt corncentration in the braze layer, and is different from that in the WC sheath. Heating for a long interval at elevated temperature may enable the cobalt concentration to simply average out, thereby degrading the performance of the cobalt based alloy in one region or the other.
The heating phase of both sintering manufacturing methods and post manufacture annealing methods can also be detrimental to the different regions of the insert. As an example, the crystalline structure of carbon on the PDC can be adversely affected by physical changes at high temperatures, whether applied in the manufacturing step or the annealing step. This reduces the wear properties of the PDC. Above a certain temperature, the carbon will begin to oxidize or otherwise be affected chemically, thereby also significantly reducing the wear properties of the PDC. Therefore, it is necessary to maintain sintering and annealing temperatures below a threshold at which damage to the PDC is incurred. Using prior art teaching, this can be accomplished by longer wintering and annealing heating times but at lower temperatures. These longer heating periods, however, result the previously discussed cobalt migration problem which, contradictorily, is minimized by heating for a shorter period of time but at a higher temperature.
Sintering and annealing at elevated temperatures for long periods of time can be detrimental to the grain size of the wear surface which can, in turn, affect the resilience of the wear surface. The smaller the grain size, the more resistant the material is to chipping and fracturing. High sintering and annealing temperatures tend to increase the grain size of sintered material and thereby degrade wear properties.
The use of a mold to fabricate wear inserts or integral wear resistant parts can be very expensive, especially if relatively small numbers of pieces are to be fabricated. An expensive mold or cast is required in the sintering of conventional alloys using high temperature-high pressure techniques while a low cost mold is need in microwave sintering of conventional alloys using methods and apparatus disclosed in the parent U.S. Patent Application.
In summary, prior art teaches the manufacture and the use of various abrasion and erosion resistant materials to form inserts which are used as wear surfaces in drill bits, and which can also be used for wear surfaces on machine tools, drill bits, bearings, and other similar surfaces. Many of the processes in the cited references require high temperatures and high pressures to sinter conventional alloys for a relatively long period of time to form the wear resistant surface material, or to bond the wear resistant surface material to the underlying support substrate, or both. A mold or cast is required. Using a composite drill bit insert as an example, cobalt can migrate between wear surface, braze layer, and insert body thereby perturbing the desired concentration of cobalt in each element of the insert. Furthermore, the bond between surface and substrate of the resulting inserts is subject to weakening due to differences in thermal expansion properties which become a factor as the device heits up during use. This can be reduced by annealing, but annealing at high temperatures over long periods of time also results in cobalt migration as discussed in the example above. Sintering and annealing heating for extended periods of time can also cause grain size growth which yields a wear surface which is quite brittle, subject to fracturing upon impact, and are in general very difficult to handle in the manufacturing process of tools employing such wear resistant surfaces. Sintering and annealing at high temperature can also adversely affect the chemical and physical properties of the wear surface. As an example, a PDC wear surface will tend to oxidize if heated at elevated temperatures. To minimize elemental migration between regions, to minimize grain growth, and to minimize damage to the wear surface, it is desirable to apply sintering and annealing heat it a relatively low temperature and for a relatively short period of time. Low pressure is also desirable from an economic and operational point of view. Low pressure and low temperature sintering of wear resistant components enable a low temperature allow and a mold or cast to be used. The fabrication of wear elements by means of low temperature-low pressure sintering of conventional and low temperature alloys, using microwave energy, without the use of a mold, are not known in the prior art.
The present invention sets out an improved alloy system with different levels of key ingredients in different regions. When bonded by heat, alloy migration in the regions is prevented, and regional differences are preserved. This enables simultaneous bonding of a PDC layer with a higher level of cobalt, an amount usually around 15% cobalt.
The WC body of the insert is alloyed with cobalt; but contrary to prior WC alloy bonding, the cobalt is not 15% or so. Rather it is in the range of about 6 to 10% cobalt. The optimum for many WC insert bodies is around 8% cobalt. The process begins with the PDC and WC ingredients in a mold compressed by packing with light pressure. The loose molded ingredients are held in the mold with minimal pressure prior to heating.
Microwave heating is preferred because it is quicker, operates at a lower temperature, and needs only minimal or no pressure, and can be done in a low pressure mold.
One object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear inserts, wherein the sintering temperature is generated by microwave energy and is below a level which inflicts adverse physical and chemical changes in components of the composite insert.
Yet another object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear inserts, wherein the heating cycle is relatively short in duration thereby preventing elemental migration between various components of the composite insert.
Still another object of the invention is to provide apparatus and methods for manufacturing sintered, composite wear surfaces, wherein the magnitude and duration of the heating phase of the sintering operation is set to minimize grain size growth in components of the composite insert.
An additional object of the invention is to provide apparatus and methods for effectively sintering low cobalt insert bodies. One benefit of the approach is reducing stress concentration at component interfaces, minimizing the migration of constituents between the components, and inhibiting grain growth within the components.
A still further object of the invention is to provide apparatus and methods for fabricating wear elements without the use of a high pressure cast or mold.